1. Field of the Invention
The present invention relates generally to light emitting devices, and, more particularly, to a structure for improving the reliability of organic and polymer light emitting devices and a method for producing same.
2. Related Art
Light emitting devices are used for many applications including electronics, communication systems, computer systems, and display systems. Light emitting devices are produced in many forms from a variety of materials using a variety of processes. Polymer and organic light emitting devices (OLED""s) are typically used in display system applications where high power efficiency (on the order of greater than 1 lumen/watt (Lm/W) with low supply voltages (on the order of 2.5 to 15 volts (V)) are particularly desirable.
Device reliability is typically the most difficult problem to overcome when fabricating and using OLED""s.
FIG. 1 is a cross-sectional view illustrating the layer construction of a typical prior art OLED 11. Transparent conducting anode 22, typically fabricated from Indium Tin Oxide (ITO) is applied over a transparent substrate 21. ITO anode 22 forms the positive terminal of device 11. Transparent substrate 21 may be, for example, glass or plastic. Over ITO anode 22 is applied one or more organic layers known to those skilled in the art as an organic stack 29. Organic stack 29 may include, for example, hole transport layer 24, electroluminescent layer 26, and electron transport layer 27. Organic stack 29 is typically not thicker than 300-500 nanometers (nm). Cathode layer 28 is applied over organic stack 29 and forms the negative terminal of OLED device 11. Cathode layer 28 is typically a metal capable of injecting electrons into organic stack 29. The cathode material is typically a metal of relatively low work function such as magnesium (Mg), cadmium (Ca), Ytterbium (Yb), lithium-aluminum (LiAl) alloys, etc.
Light is generated in organic stack 29 by the recombination of holes injected from the ITO anode 22 and electrons from cathode 28. The generated light exits through the transparent ITO anode 22 and the transparent substrate 21 in the direction illustrated by the arrow. Cathode 28 is reflective and acts as a mirror reflecting light towards the substrate. For convention, we will assume that the substrate 21 is at the bottom and the cathode 28 is at the top of device 11.
In order for an OLED, such as that described above, to operate at low voltages (i.e., between 2.5 and 15V) and high power efficiency (i.e., greater than 1 Lm/W), the organic stack is usually less than 350 nm thick, and typically between 150-200 nm thick. This desirable thickness poses many device fabrication challenges. Particularly, any imperfection in the device structure can cause the cathode to be in direct contact (or very close proximity) with the anode. This condition results in an area of much lower resistance than the rest of the stack and is typically referred to as a xe2x80x9cshortxe2x80x9d.
A short in a single pixel device can result in an inoperative device, while a short in a passive addressing x-y pixelated device may result in several types of cross-talk depending upon the manner in which the device is driven. Shorts are currently the primary reason for low fabrication yields in OLED technology. Shorts may occur in any of the layers forming the OLED device and may be caused by substrate imperfections, ITO layer irregularities, organic film non-uniformity, handling, etc.
OLED""s with thicker organic layers (approx. 1 micron or thicker) have been fabricated, such as xe2x80x9celectrochemical cell OLED""sxe2x80x9d, and the symmetrically configured ac light emitting (SCALE) OLED. These devices however, have other disadvantages such as slower turn-on times for electrochemical cell OLED""s, and higher AC voltage requirements for SCALE OLED""s.
In the past, polyaniline (PANI, the polymeric form of aniline) and PDOT (a type of polythiophene, which is a conducting polymer) have been used as buffer layers between an ITO layer and an organic stack in order to improve efficiency and reliability. Both of these materials are conductive polymers when combined with a xe2x80x9cdopantxe2x80x9d, such as a strong acid or a poly acid (polystyrene sulfonate). The acid effectively xe2x80x9cchargesxe2x80x9d positively the backbone of PANI or PDOT, thus making their electronic structures suitable for charge conduction. While used to some degree of success in reducing the occurrence of shorts when applied over an ITO layer, both PANI and PDOT are very dark in color. This implies that they are intrinsically strongly absorbing, therefore requiring that their thickness be kept below approximately 100 nm to 150 nm if they are to be placed upon an ITO layer (the transparent electrode). Such a thin layer is difficult to fabricate in large scale and will be ineffective at suppressing shorts over large defect areas. Furthermore, PANI and PDOT as currently available are not completely compatible with the solvents and solutions used in current microlithography photoresist methods of patterning OLED""s.
Thus, an unaddressed need exists in the industry for a process compatible organic light emitting device that can be fabricated using a current self-limiting layer that is greater in thickness than a few hundred nanometers, operates at low voltages and provides high power efficiency.
The invention provides an organic light emitting device that operates at low voltage, has high power efficiency, and is simple to fabricate using available techniques. Although not limited to these particular applications, the structure to improve the reliability of organic and polymer light emitting devices and method for producing same is particularly suited for organic light emitting devices. The devices can be fabricated using a process by which the materials comprising the device are vapor deposited into amorphous films, or cast from solutions.
In architecture, the present invention can be conceptualized as an organic light emitting device including an electrode, a current self-limiting structure and an organic stack located between them. The current self-limiting structure resides in contact with the electrode.
In a first alternative embodiment, the current self-limiting structure resides between an electrode and an additional conducting layer.
In a second alternative embodiment of the present invention, the current self-limiting structure is applied as a patterned lattice structure over an electrode.
In yet another alternative embodiment, the current self-limiting structure is applied as a grid, defining windows within which an electrode of the light emitting device may be applied.
The present invention may also be conceptualized as providing a method for increasing the reliability of an organic light emitting device, comprising the following steps.
An organic light emitting device having increased reliability is formed with a current self-limiting structure placed within the organic light emitting device. The current self-limiting structure is formed in continuous contact with an electrode of the organic light emitting device, or can be formed as a grid, or patterned lattice, in contact with an electrode of the organic light emitting device.
The invention has numerous advantages, a few of which are delineated, hereafter, as merely examples.
An advantage of the invention is that it provides light output using a low voltage power supply.
Another advantage of the invention is that the light emitting device operates at high power efficiency.
Another advantage of the invention is that it lends itself to generally available simple fabrication techniques.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.